In recent years, the increase in performance of diesel engines has enabled oil mixture to be used as fuel other than light oils or heavy oils, which are used alone. The oil mixture refers to a type of oil made by mixing multiple types of oils having different viscosities. Accordingly, inexpensive low-quality oil having a viscosity higher than that of high-quality oil such as light oil has been selected as a material of the oil mixture. This reduces fuel costs.
For such an oil mixture, not only oils having low viscosities, but also oils having high viscosities are used as materials, taking into consideration the prices of the fuels. To use oil mixture as fuel, the viscosity of the oil mixture itself is adjusted. For example, Patent Document 1 discloses a fuel supply apparatus for supplying oil mixture as described above to an engine. In this fuel supply apparatus, the viscosity of oil mixture, which has been generated by mixing various oils, is detected in a supply passage for supplying the oil mixture to an engine. Based on the detection result, the oil mixture is heated in the supply passage leading to the engine to adjust the viscosity of the oil mixture, such that the viscosity becomes appropriate for use in a diesel engine.
However, in the above fuel supply apparatus, the oils constituting the oil mixture can include components that can be oxidized and degraded by heating the oil mixture to adjust the viscosity. Particularly, in recent years, biodiesel fuel, which is obtained by refining plant-derived oil and waste edible oil, has received a lot of attention as a circulation type energy. Use of oil mixture obtained by mixing these oils with light oil or heavy oil has been proposed. However, in general, biodiesel fuel is easily oxidized and degraded by heat. Therefore, use of such an oil mixture as fuel can cause problems such as a lowered fuel efficiency.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-127279